1. Field of the Invention
This invention relates to fine particle size aluminum powder compositions, and to processes for their manufacture. The compositions of this invention are particularly useful as sensitizers in the well-known aqueous slurry type blasting agents.
2. Description of the Prior Art
Fine particle size aluminum powders, wherein the particles are of sizes down to only a few microns, a size range of from about 2 microns to about 10 being typical, have a number of uses. For example, they are used in paints, printing inks and plastics as colorants. They are also used in aqueous slurry type blasting agents as sensitizers.
However, all uses of aluminum particulate materials which involve handling a particulate material containing at least a proportion of fine powder of the above-mentioned size range require that a major hazardous property of such fine powders be borne in mind: micron size aluminum powder is a highly reactive material which is prone to dust explosions. This very real and dangerous hazard has, and still does, place significant constraints upon the commercial use and handling of such powders. The risk is considerably higher if the operation in question both involves the presence of micron-size powder and requires the handling, at some stage, of dry powder.
These problems can be either mitigated sufficiently or even effectively avoided, in many uses by ensuring that the aluminum powder is always in what is called a "wetted down" condition: that is, it is coated at all times with an organic liquid with which the aluminum powder does not react. Generally the organic liquid used is a hydrocarbon, the most frequently used ones being mineral spirits or kerosene.
It is to be noted, that water generally cannot be used as the wetting-down liquid. The reason for this is that fine powder aluminum reacts even with cold water, to form hydrogen and a more, or less, hydrated aluminum oxide. The precise nature of the aluminum oxide product depends on the precise reaction conditions. Indeed this property is deliberately relied upon in some uses, for example in blowing concrete and cement systems to produce voids in them during the setting process.
Whilst these wetted-down pastes are less hazardous to handle from the explosion aspect, due to the elimination of free airborne dust, they are not a complete solution to the problem. There are two main reasons for their being not completely satisfactory.
The first is that one hazard has, to an extent, merely been replaced by another: the dust explosion hazard has been overcome at the expense of creating a flammability hazard due to the hydrocarbon liquid. Whilst this flammability too can be mitigated by a careful choice of the hydrocarbon it cannot be totally eliminated. It should also be borne in mind that should a fire result then the presence of the aluminum powder exacerbates the problems since it too, will join in the conflagration.
The second is that in many systems the presence of the hydrocarbon used to wet-down the aluminum powder cannot be tolerated, or at best only tolerated to a limited extent. It usually cannot be tolerated due to the difficulties its presence causes when the wetted-down powder is put to its intended end-use. This difficulty is frequently acute in water-containing systems, especially those containing little, or no, other organic solvents. Further, the dispersal of the hydrocarbon into such an aqueous system by the use of surface active agents is not always a complete cure to the problems. Representative aqueous systems in which only limited amounts of hydrocarbons can be tolerated are concrete and cement foaming systems, and slurry blasting agents. In both cases a safe and easily handleable powder composition which will disperse into water is desirable. Of these two types, slurry blasting agents are perhaps the more tolerant of hydrocarbons.
But aqueous slurry type blasting agents also impose limitations on the aluminum powder which, to an extent, are virtually incompatible and one of which constitutes a severe disadvantage in a cement or concrete foaming system. On the one hand the powders are required to be sufficiently chemically reactive to play an effective part in the explosion processes. But on the other hand the powders are required to be sufficiently chemically unreactive toward water. The reason for the first of these mutually opposing criteria is obvious: a sensitizer that does not work is no use. The second arises for reasons that are not so obvious. If in the aqueous ammonium nitrate-based systems commonly used in slurry-type blasting agents any aluminum powder used as sensitizing reagent reacts with the water in the system then some, or all, of three major consequences follow. First, all of the sensitizer could get consumed, resulting in a blasting agent which will not fire. The dangers that can thereafter follow need no comment. Second, consumption of only part of the sensitizer can have two results: the formation of hydrogen gas bubbles and the coating of the sensitizer particles with a layer of some sort of hydrated oxide. These gas bubbles again cause hazards: in an extreme case they can result in a slurry either going "dead" and not exploding at all, or in a less extreme case severely inhibit the explosion processes. Either way, the blasting agent does not function properly. The generation of an oxidic coating on the particles is also deleterious to blasting agent performance: It is known from studies of air-blown particulate aluminum used as a fuel in slurry-type blasting agents that particles having a heavily oxidised surface do not work at all well. But the third reason is a more practical one. In a mine a bore hole is not necessarily blown immediately after filling with blasting agent. The delay can be hours or even days. A mine operator therefore requires a system which can accomodate such a delay: clearly a system in which the aluminum sensitizer powder reacts cannot accomodate much delay between charging the bore hole and firing it.
But nevertheless against all of these disadvantages must be set the fact that industrially the most convenient procedure for preparing fine particle size aluminum is to grind it, commonly in a ball-mill, and in a hydrocarbon solvent, such as mineral spirits or kerosene. It therefore follows that it is desirable to develop procedures whereby a safe to handle powder that does not, especially, present dust explosion problems, can be obtained.
It has been proposed to overcome these dust hazards for such powders when dry, and with blasting agent use in mind, by coating the aluminum powder. In one procedure the aluminum dust is coated with a polyfluoroethylene material, such as Teflon-K (Trade Mark) by tumbling the dry aluminum powder with the polymer at a temperature of about 100.degree. C. It can immediately be seen that this method involves handling a dry aluminum powder. This technique is, however, successful in that it will effectively de-dust powder material such as vacuum dried aluminum flake. But this process also has two further disadvantages. First, the polyfluoroethylene polymer materials are relatively expensive. In a blasting agent context its second disadvantage is far more important: these polyfluorethylene coatings markedly reduce the effectiveness of aluminum powders as sensitizing agents. It can become necessary to use up to 30% more de-dusted aluminum carrying such a polyfluoroethylene coating in order to achieve the same effect as the un-coated material in a slurry-type blasting agent.
In an alternative procedure it is proposed to grind, for example in a ball mill, blown aluminum powder in the absence of water and in the presence of stearic acid and a hydrocarbon medium. The hydrocarbon is commonly mineral spirits or kerosene. The product is then separated from the hydrocarbon by filtering and vacuum drying. Such a process is not completely satisfactory. First and foremost the product still contains some dust and therefore the handling hazard is only decreased, not eliminated. Second, these powders have been found on occasions to gas, that is, to react with the water in the blasting agent slurry. The undesirability of this has been discussed above. But third their performance is severely impaired if any hydrocarbon remains on the powder: it is frequently the case that such impairment is also only discovered when a charged borehole either fails to explode, or does not blow properly. Again these hazards have been discussed above.
Thus neither of these procedures provide a product that is commercially attractive for field use in a slurry-type blasting agent.